Retroreflectors comprise three reflecting flat faces in mutually perpendicular planes, related so that an incoming beam is redirected along a reverse path. The resolution of a retroreflector suffers in case its reflecting faces are not exactly mutually perpendicular. Hollow retorreflectors have separate reflecting faces, and these can be adjusted in relation to each other during assembly. As error detector is used to check the accuracy of the assembly. The error detector directs its beam toward the retroreflector, and the resulting image(s) produced in the error detector is used to guide the adjustment of the parts.
In case of very large retroreflectors, especially those designed to offset the path of the outgoing reflected beam in relation to the incoming beam, the aperture of the error detector may be inadequate.